Method for On-Line Monitoring of Cable Insulation by Reverse Injection of Low-Frequency Signal

Abstract

The invention discloses a method for on-line monitoring of cable insulation through reverse injection of low-frequency signals. Low-frequency voltage signals are injected through the open triangle side of an electromagnetic voltage transformer connected to a busbar, and windowed interpolation improved by Fast Fourier is used. The algorithm processes the current signal of the collected cable ground wire and the voltage signal of the cable core to obtain the low-frequency dielectric loss factor of the cable, and judge the aging degree of the cable, so as to realize the active online monitoring of cable insulation aging.

Description

technical field [0001] The invention belongs to the field of electric technology and relates to an online monitoring technology of power cables, in particular to a method for online monitoring of cable insulation through reverse injection of low-frequency signals into a power grid. Background technique [0002] With the development of urbanization, power cables are widely used because of their good electrical properties. At present, the service life of many cables in operation is close to its limit of 30 years. Knowing the insulation status of these cables is the basis for ensuring the stable operation of the power system. However, it is difficult to find out the failure of the cable insulation state in time through regular maintenance, and it is difficult to meet the reliability requirements of the power system. Realizing the on-line monitoring of the cable insulation state is an inevitable trend in the development of the power system. [0003] Traditional on-line monitori...

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Problems solved by technology

(1) The DC component method uses the leakage current rectified by the water branch of the cable as the criterion for the aging degree, and is not suitable for cables with insulation aging caused by other reasons (such as thermal aging)

(2) DC superposition method, apply a 50V low-voltage DC voltage to the cable through the neutral point of the voltage transformer, measure the weak DC current or its insulation resistance through the cable insulation layer to judge the aging of the cable insulation; the main method of this method The disadvantage is that the DC current passing through the voltage transformer for a long time will cause the magnetic circuit of the transformer to saturate and generate zero-sequence voltage, which will cause the relay of the substation to malfunction.

(3) AC superposition method, superimpose 2 times the power frequency frequency plus 1Hz AC voltage on the running cable shielding layer, and detect the 1Hz AC characteristic current signal to judge the aging degree of the cable; but for the network where the neutral point is directly grounded , the power supply cannot be directly superimposed on the core, so the AC superposition method is not applicable

(4) Partial discharge method, apply a higher voltage to the main insulation of the cable, measure the partial discharge caused by the impurities, burrs, air gaps, etc. of the cable under the joint action of electric field, heat, mechanical and other factors, and respond according to the size of the discharge signal The damage of the main insulation; the partial discharge method is the most widely used method at present, but the partial discharge signal is complicated and it is difficult to achieve online monitoring

These traditional methods are not ideal enough to meet the requirements of the power system, and it is difficult to realize the online monitoring of the operation site. It is necessary to explore new online monitoring methods

[0004] In recent years, scholars in related fields have proposed some new methods for on-line monitoring, for example: (1) It is proposed to install an on-line monitoring device in the cable tunnel, and conduct on-line monitoring of high-voltage cable faults through the sheath current measured on the coaxial cable connection line and diagnosis, since the monitored current transformer is directly installed on the connection line of the interconnection box, the difficulty of the current research is to correctly separate the leakage current and the sheath circulation; A current transformer is installed at both ends of the phase cable to measure the current flowing through the cable head end and the end core to obtain the instantaneous value of the leakage current, and then use the monitored value as the criterion for the insulation state. However, in practical applications , the current flowing through the cable head end and the end wire core is difficult to measure synchronously, and there will still be errors, and the method of adding two current transformers CT is not practical enough, and the applicability is not strong; (3) It is proposed to use the system power disturbance Under the current transient response, the overall aging state of the cable can be identified by calculating the insulation admittance spectrum, but the disturbance of the power system caused by motor startup, line switching, short-circuit faults, etc. is uncontrollable, so this method has certain limitations

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